The present application generally relates to exhaust aftertreatment systems, and more particularly, but not exclusively, to selective catalytic reduction (“SCR”) systems. Presently available SCR systems adsorb ammonia (NH3) on a catalyst and then react the NH3 with NOx to reduce the NOx emissions. The NH3 is typically stored as a less reactive composition, e.g. urea, and hydrolyzed into NH3 in the exhaust system as required to reduce the NOx emitted by the engine. At certain system operating conditions, the NH3-based NOx reduction mechanism breaks down due to lack of NH3 storage capacity on the SCR catalyst, due to an inability to rapidly hydrolyze urea into NH3, or for other reasons. When the NH3-based NOx reduction mechanism breaks down, the net NOx emissions of the system increase.